Exemplified by laser emission spectroscopy and spark emission spectroscopy, emission spectroscopy serves for material analysis, whereby the laser beam or the sparks are used for the vaporization of minimal amounts of material, so that the vaporized material can be analyzed. According to FIG. 1A at (1) a laser beam is focussed on the workpiece to be analyzed. Based on the natural absorption of the laser beam by the workpiece 4 an energy coupling as per FIG. 1B occurs at 2 in a localized area, wherefrom a part of the material is vaporized according to FIG. 1A-1D, the vapor being seen at 3. The amount of the vaporized material depends on the energy, the output and the output density, the local output distribution and the wavelength of the laser beam. The generation and the state of the vaporized material are also influenced by the characteristics themselves and by the surrounding atmosphere.
The laser beam is coupled into the vaporized material, so that the latter is brought into a plasma state or a plasma-like state. As a result of the energy coupling, the components of the plasma are induced to emit radiation. The thereby emitted radiation 40 according to FIG. 1D is characteristic of the composition of the material to be analyzed.
The beam 40 emitted by the plasma 6 is directed towards a spectrometer and is there spectrally decomposed. For instance the dependency of the radiation intensity I.sub.S =f(.lambda.) can be obtained as shown in FIG. 3. This information is fed to a processing unit, which for instance is equipped with a computer and an output device. In the processing unit, intensity, or line ratios, are calculated with the aid of calibrating curves, in order to determine the element concentration of the material. For instance a ratio between the intensity I.sub.1 of an emission line .lambda..sub.1 in relation to the intensity I.sub.2 of an emission line .lambda..sub.2 is established, whereby the emission line .lambda..sub.2 serves as reference line and for instance originates from the element which is most often present in the examined material. With the aid of the ratio I.sub.1 /I.sub.2 it is possible to determine in percentage the content C.sub.1 of a first element of the entire material with the aid of a calibration curve I.sub.1 /I.sub.2 =f(C) according to FIG. 3a.
Such material analyses with laser beams are contactless methods, which can practically be carried out without sample taking. Being performed with fast components, they afford the possibility of high measuring speeds. Therefore they can be incorporated in processing and machining production lines, without impairing the flow of workpieces by taking samples and transporting the samples to the analyzing device. It is possible to perform elemental and multi-elemental analyses. Through on-line analysis, it is possible for instance to survey quantitatively and qualitatively the production processes of workpieces. Furthermore it is even possible to intervene in a controlled manner in the production of workpieces, when high measuring speeds make possible an on-line analysis. For instance it is possible to control the introduced raw materials, to test the identity of workpieces and to sort out mixed materials and workpieces. However these basic possibilities can be limited by the fact that accuracy of measurement and the reproducibility are low because the changes in the plasma state with time are not sufficiently considered. FIG. 2 illustrates the principle that I=f(t) represents the timed interdependence between the path of the curve of the intensity of the laser beam and the path of the curve of the intensity of the plasma-emitted beam. For instance to a laser beam intensity of I.sub.L1 (t) the intensity I.sub.S1 (t) is supposed to correspond. It can be seen that the emitted radiation occurs with a delay, because the plasma has first to build up under the effect of the laser beam. If regarding the curve of its intensity the laser radiation behaved according to I.sub.L2 (t), so for the intensity of the emitted plasma radiation a curve according to I.sub.S2 (t) would result. The different intensity curves of the emitted radiation are explained by the fact that the temperature conditions responsible for the intensity of the emitted radiation undergo a delayed change, depending on the irradiating energy and thereby by the pulse course of the laser beam. For this reason it is important to establish precisely the time range or the particular moment in time t.sub.m for the measurement or the evaluation because only at that particular moment is the radiation emission optimal. Reference is made to FIGS. 4a to 4c, wherein the intensity curve I.sub.S =f(.lambda.) is represented, and namely for the moments in time t.sub.a, t.sub.m and t.sub.e. At the moment t.sub.a the radiation intensity is relatively low, the same applying also to I.sub.te the moment t.sub.e. This is in correspondence with I.sub.S1 (t). It can be seen that the emission line is only weakly developed. By contrast the emission lines at the moment t.sub.m are optimally developed and the analyses according to FIGS. 3, and 3a can be carried out with maximal precision. However, in the known processes this is considered only to the extent that after the start of the laser pulse or of a discharge process triggering the laser pulse a fixed moment in time is established, so that different developments which the plasma undergoes in time are not taken into consideration. Therefore in the known devices the optimal point in time t.sub.m for different plasmae is not reached.
From DE-A-34 13 589 a device for laser emission spectroscopy is known, wherein a fraction of the radiation emitted by the material to be analyzed is directed to a photodiode, by means of which a reference signal is produced which is fed to a signal processor. Here the signal of the photodiode, which measures only the integral radiation intensity of the plasma, serves for the standardization of the signal spectrum found by the spectrograph. A different development of the plasmae over time, and therewith an increase of the accuracy of measurement, as well as a reduction of the measuring times does not take place.
From U.S. Pat. No. 4,690,558 a process for the laser emission spectroscopy is known, wherein the accuracy of the analysis is supposed to be enhanced due to the fact that GAUSS-distribution of the TEM.sub.00 -Mode is used. Further a measuring of the spectral intensity is supposed to be performed only when the intensity ratio of a pair of preselected spectral lines of the light emitted by the object to be examined lies within a predetermined range. Consequently the described accuracy errors are reduced, however there is no reduction in the measuring times. A way to influence the way the laser works is not described.
From WO-A-86/00552 a device for the processing of workpieces by means of laser beams is known, based on the buildup of a plasma to be kept within limits. The device has diagnostic means, which serve for the constant detection of the processes developing in the area where the workpiece is being processed and for the detection of plasma parameters. With these plasma parameters the laser intensity is kept within predetermined limits, in order to avoid a plasma detonation. A quantitative spectrographic analysis of the material does not take place.